721691 has 4 divisors (see below), whose sum is σ = 733584. Its totient is φ = 709800.

The previous prime is 721687. The next prime is 721697. The reversal of 721691 is 196127.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 196127 = 29 ⋅6763.

It is a cyclic number.

It is not a de Polignac number, because 721691 - 2² = 721687 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 721691.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 721691.

It is not an unprimeable number, because it can be changed into a prime (721697) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 5855 + ... + 5976.

It is an arithmetic number, because the mean of its divisors is an integer number (183396).

2⁷²¹⁶⁹¹ is an apocalyptic number.

721691 is a deficient number, since it is larger than the sum of its proper divisors (11893).

721691 is a wasteful number, since it uses less digits than its factorization.

721691 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 11892.

The product of its digits is 756, while the sum is 26.

The square root of 721691 is about 849.5239843583. The cubic root of 721691 is about 89.6982072508.

It can be divided in two parts, 7216 and 91, that multiplied together give a palindrome (656656).

The spelling of 721691 in words is "seven hundred twenty-one thousand, six hundred ninety-one".